What I learned from my first serious attempt low-level Haskell
simonmarhaskell at gmail.com
Thu Apr 5 03:52:52 EDT 2007
Stefan O'Rear wrote:
> 2. Parameters are very expensive. Our type of functions that build
> (ignoring CPS for the time being) was MBA# -> Int# -> [ByteString],
> where the Int# is the current write pointer. Adding an extra Int#
> to cache the size of the array (rather than calling sMBA# each
> time) slowed the code down ~2x. Conversely, moving the write
> pointer into the byte array (storing it in bytes 0#, 1#, 2#, and
> 3#) sped the code by 4x.
If you were measuring on x86 then parameters are passed on the stack, which may
be expensive. On x86_64 the first 3 arguments are passed in registers, which is
usually a win, but if the function immediately does an eval they need to be
saved on the stack anyway. Still, 4x sounds like a lot, perhaps you managed to
avoid a stack check in the inner loop or something.
> 3. MBA# is just as fast as Addr#, and garbage collected to boot.
Not really surprising, that.
> 4. You can't keep track of which version of the code is which, what is
> a regression, and what is an enhancement. Don't even try. Next
> time I try something like this I will make as much use of darcs as
Absolutely - if you'd used darcs, then we could peer in more detail at changes
that you thought gave counter-intuitive results.
Simon Peyton-Jones wrote:
> | 5. State# threads clog the optimizer quite effectively. Replacing
> | st(n-1)# with realWorld# everywhere I could count on data
> | dependencies to do the same job doubled performance.
> The idea is that the optimiser should allow you to write at a high level, and do the book keeping for you. When it doesn't, I like to know, and preferably fix.
> If you had a moment to boil out a small, reproducible example of this kind of optimisation failure (with as few dependencies as poss), then I'll look to see if the optimiser can be cleverer.
Yes, and *please* add some of this folklore to the performance wiki at
http://haskell.org/haskellwiki/Performance, if you have the time.
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